Segmentation fault (core dumped) while using semaphores in c

Question

I am trying to write a code, which output should create a molecule. Always two hydrogen will be taken and one oxygen and then one molecule will be created. There is still so much to do so many of variables you probably wont understand but I cant move because I get segmentation fault all the time I try to do something with semaphores, and I don't know if I am doing something wrong in initializing them. Or I am doing it badly and there is some other more efficient way?

#include <sys/types.h>
#include <sys/sem.h>
#include <sys/ipc.h>
#include <unistd.h>
#include <pthread.h>
#include <time.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <semaphore.h>
#include <sys/shm.h>
#include <sys/wait.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <stdbool.h>

typedef struct parametry{
    int NO; // pocet kysliku
    int NH; // pocet vodiku
    int TI; // Maximální čas milisekundách, po který atom kyslíku/vodíku po svém vytvoření čeká, než se zařadí do fronty na vytváření molekul. 0<=TI<=1000
    int TB; // Maximální čas v milisekundách nutný pro vytvoření jedné molekuly. 0<=TB<=1000
} Tparams;

FILE *file;

sem_t *hydrogen;
sem_t *oxygen;
sem_t *barrier_1;



int param_write(int argc, char **argv, Tparams *parametry)
{
    bool wrong_arguments = false; // pokud jsou argumenty spatne vykona exit
    if(argc > 5)
    {
        fprintf(stderr, "Moc argumentu\n");
        exit (1);
    }
    if(argc < 5)
    {
        fprintf(stderr, "Malo argumentu\n");
        exit (1);
    }
    parametry->NO = atoi(argv[1]);
    parametry->NH = atoi(argv[2]);
    parametry->TI = atoi(argv[3]);
    parametry->TB = atoi(argv[4]);

    if(parametry->NO <= 0)
    {
        fprintf(stderr, "Spatne zadany argument NO\n");
        wrong_arguments = true;
    }
    if(parametry->NH <= 0)
    {
        fprintf(stderr, "Spatne zadany argument NH\n");
        wrong_arguments = true;
    }
    if(parametry->TI < 0 || parametry->TI > 1000)
    {
        fprintf(stderr, "Spatne zadany argument TI\n");
        wrong_arguments = true;
    }
    if(parametry->TB < 0 || parametry->TB > 1000)
    {
        fprintf(stderr, "Spatne zadany argument TB\n");
        wrong_arguments = true;
    }
    if(wrong_arguments)
    {
        exit (1);
    }

    return 0;
}

int file_control()
{
    if((file = fopen("proj2.out", "w")) == NULL) 
    {
        fprintf(stderr, "Chyba s suborem\n");
        exit(1);
    }
    return 0;
}

void hydrogen_process(int idH,int *hyd_count)
{
    idH++;
    sem_wait(hydrogen);
    printf("H %d: started\n", idH);
    printf("H %d: going to queue\n", idH);
    (*hyd_count)++;
    if((*hyd_count) == 2)
    {
        sem_post(oxygen);
        (*hyd_count) = 0; 
    }

    
}

void oxygen_process(int idO)
{
    sem_wait(oxygen);
    printf("O %d: started\n", idO);
    printf("O %d: going to queue\n", idO);
    printf("Molecule created\n");
    sem_post(hydrogen);
    sem_post(hydrogen);
}

int main (int argc, char **argv)
{
    Tparams parametry;
    file = fopen("proj2.out","w");
    param_write(argc, argv, &parametry);
    file_control();

    pid_t pid_process;
    pid_t pid_ox;
    pid_t pid_hyd;
   

    sem_init(hydrogen, 1, 2);
    sem_init(oxygen, 1, 0);
    sem_init(barrier_1, 1, 1);

    int hyd_count = 0;
   
    pid_process = fork();
    if(pid_process < 0)
    {
        fprintf(stderr, "Chyba pri volani funkce fork");
        fclose(file);
        exit (1);
    }
    else if(pid_process == 0) // child
    {
        for(int i = 0; i < parametry.NO;i++)
        {
            usleep((random() % (parametry.TI + 1))*1000);   
            pid_ox = fork();
            if(pid_ox < 0) 
            {
                fprintf(stderr, "Chyba pri volani funkce fork");
                fclose(file);
                exit (1);
            }
            if(pid_ox == 0)
            {  
                oxygen_process(i);
                exit (0);
            }else
            {
               
            }
        }
    }else if(pid_process > 0) // parent
    {
        for(int i = 0; i < parametry.NH;i++)
        {
            usleep((random() % (parametry.TI + 1))*1000);
            pid_hyd = fork();
            if(pid_hyd < 0) 
            {
                fprintf(stderr, "Chyba pri volani funkce fork");
                fclose(file);
                exit (1);
            }else if(pid_hyd == 0)
            {
                hydrogen_process(i, &hyd_count);
                exit (0);
            }else
            {
                 
            }
        }
    }
    sem_destroy(hydrogen);
    sem_destroy(oxygen);
    sem_destroy(barrier_1);
 
    fclose(file);
    return 0;
}

Your semaphores are not allocated. You are passing NULL pointers to the sem_* functions. Look around for some usage examples. — Eugene Sh.
– Eugene Sh., Commented Apr 26, 2022 at 13:57

th33lf · Accepted Answer · 2022-04-26 14:11:49Z

1

Seg faults are probably due to the fact that you are using uninitialized pointers to sem_t, with no memory allocated for them to point to. sem_init doesn't allocate memory for the sem type itself.

I'd start with:

sem_t hydrogen;
sem_t oxygen;
sem_t barrier_1;

Then pass around references to these in your sem_ API. Example:

sem_init(&hydrogen, 1, 2);

edited Apr 26, 2022 at 14:11

answered Apr 26, 2022 at 14:02

th33lf

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